Production of pervious low density carbon fiber reinforced composite articles

ABSTRACT

The efficient production of pervious low density carbon fiber reinforced composite articles is made possible through the use of the present invention. A fibrous open weave tape (as described) which is capable of undergoing conversion to a carbonaceous fibrous material is continuously passed in the direction of its length through a series of heating zones to form a fibrous carbon tape (preferably of graphitic carbon) wherein an open weave construction is maintained. At least one layer of the resulting fibrous open weave carbon tape may be impregnated with a thermosetting resinous material, and subsequently cured to form a pervious composite article. The composite articles of the present invention exhibit a high specific modulus and strength, and are extremely lightweight. The lightweight composite articles may be utilized as structural elements, and are particularly suited for use as a facing sheet of an acoustic sandwich liner which serves as a noise suppression function in jet engines.

United States Patent [1 1 [111 3,779,789 Park Dec. 18, 1973 [541PRODUCTION OF PERVIOUS LOW DENSITY CARBON FIBER REINFORCED COMPOSITEARTICLES Primary Examiner--Wi1liam D. Martin Assistant Examiner-M.Sofocleous Att0rneyThomas J. Morgan, Charles B. Barris and J. A. Shedden5 7 ABSTRACT The efi'icient production of pervious low density carbonfiber reinforced composite articles is made possible through the use ofthe present invention. A fibrous open weave tape (as described) which iscapable of undergoing conversion to a carbonaceous fibrous material iscontinuously passed in the direction of its length through a series ofheating zones to form a fibrous carbon tape (preferably of graphiticcarbon) wherein an open'weave construction is maintained. At least onelayer 'of the resulting fibrous open weave carbon tape may beimpregnated with a thermosetting resinous material, and subsequentlycured to form a pervious composite article.

The composite articles of the present invention exhibit a high specificvmodulus and strength, and are extremely lightweight. The lightweightcomposite .articles may be utilized as structural elements, and areparticularly suited for use as a facing sheet of an acoustic sandwichliner which serves as a noise suppression function in jet engines.

9 Claims, 3 Drawing Figures [75] Inventor: Im Keun Park, Summit, NJ.

[73] Assignee: Celanese Corporation, New York,

[22] Filed: Apr. 20, 1971 [21] App]. No.: 135,698

[52] 11.8. C1 1l7/46 CB, 117/47 R, 117/115, 117/98, 156/327, 161/72,161/89 [51] Int. Cl B44d 1/092 [58] Field of Search 1 17/46 BC, 47 R,117/115; 264/29; 23/2091; 156/327; 161/72,

[56] References Cited UNITED STATES PATENTS 3,285,696 l/l966 Tsunoda23/2091 3,399,252 8/1968 Hough et a1. 23/2093 3,449,077 6/1969 Stuetz23/209.1 3,508,874 4/1970 Ru1ison.... 23/2091 3,528,774 9/1970 Ezekiel23/2091 3,540,848 11/1970 Krugler et a1... 23/2091 3,539,295 11/1970 Ram23/2091 3,547,584 12/1970 Santangelo 23/2091 3,552,923 1/1971 Carpenteret a1... 23/2091 3,656,910 4/1972 Ferment 423/447 K n A. p .1 r

m qz- :i

7 B L L L t L PRODUCTION OF PERVIOUS LOW DENSITY CARBON FIBER REINFORCEDCOMPOSITE ARTICLES BACKGROUND OF THE INVENTION In the search for highperformance materials, considerable interest has been focused uponcarbon fibers. The terms carbon fibers or carbonaceous fibers are usedherein in the generic sense and include graphite fibers as well asamorphous carbon fibers. Graphite fibers are defined herein as fiberswhich consist essentially of carbon and have a predominant X-raydiffraction pattern characteristic of graphite. Amorphous carbon fibers,on the other hand, are defined as fibers in which the bulk of the fiberweight can be attributed to carbon and which exhibit an essentiallyamorphous X-ray diffraction pattern. Graphite fibers generally have ahigher Youngs modulus than do amorphous carbon fibers and in additionare more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected tomake substantial utilization of fiber reinforced composites, andgraphitic carbon fibers theoretically have among the best properties ofany fiber for use as high strength reinforcement. Among these desirableporperties are corrosion and high temperature resistance, low density,high tensile strength, and high modulus.

As is known in the art, numerous procedures have been proposed in thepast for the conversion of various organic polymeric fibrous materialsto a carbonaceous form while retaining the original fibrousconfiguration essentially intact. Such procedures have in common thethermal treatment of the fibrous precursor in an appropriate atmosphereor atmospheres which is commonly conducted in a plurality of heatingzones, or alternatively in a single heating zone wherein the fibrousmaterial is subjected to progressively increasing temperatures. Bothbatch and continuous processing techniques have been proposed. From thecommercial standpoint those processes which are capable of functioningon a continuous basis are generally considered to be the mostattractive. However, many of the prior art continuous conversiontechniques have been inherently limited to the processing of a singleend of fibrous precursor at a given time. Such techniques while offeringthe advantages of possible automation, still suffer the disadvantage oflimited productivity.

Techniques have heretofore been proposed for the simultaneous conversionof a substantial number of fibrous ends to a carbonaceous form whichhave involved the thermal treatment of a fibrous precursor while in theform of a woven cloth. See, for instance, Belgian Pat. Nos. 720,947 and726,761, as well as U. S. Pat. No. 3,541,582 for representativedisclosures of the processing of woven cloth precursors. However, thefiber bundles present in the conventionally woven carbon cloths commonlypossess at least some permanent crimp at the warp and weft cross-overpoints and the single filament tensile properties of the fibers presentwithin the cloths have tended to be adversely influenced.

There has arisen in the advanced engineering composite art the need foran efficient technique to produce pervious carbon fiber reinforced highstrength composite articles of extremely low density. Woven carbonfabrics or cloths wherein weaving of a fibrous precursor was conductedprior to thermal conversion have been unsuitable for use in suchapplications because of (a) the high fiber density within the same and(b) impaired tensile properties resulting from fiber crimp. Priorattempts at the production of pervious low density carbon fiberreinforced composites have involved the tedious weaving of previouslycarbonized fiber bundles to form a substantially balanced cloth of anopen weave construction which is subsequently resin impregnated with amatrix material. Such weaving by necessity must be conducted at arelatively slow rate because of the fragile nature of the previouslycarbonized fiber bundles. Even if such special weaving techniques areutilized, difficulties have arisen, however, with respect to the qualityof fibrous reinforcement since the carbonized fiber bundles tend to bereadily damaged during weaving with a concomitant diminution of theirtensile properties.

It is an object of the invention to provide an improved process for theformation of pervious low density carbon fiber reinforced compositearticles.

It is an object of the invention to provide a novel low density carbonfiber reinforced composite article comprising at least one layer or plyof a highly directional woven carbon fiber tape having an improved openweave construction which is impregnated with a substantially curedthermosetting resinous material.

It is an object of the invention to provide an improved process forforming a woven carbon fiber tape possessing an open weave constructionsuitable for use as a fibrous reinforcing medium in a pervious lowdensity composite article.

It is an object of the invention to provide a novel carbon fiber tape ofa highly directional open weave construction which is suitable for useas a fibrous reinforcing medium in a pervious low density compositearticle.

It is another object of the invention to provide improved pervious lowdensity carbon fiber reinforced composite articles exhibiting superiortranslation of fiber properties into composite properties.

It is another object of the invention to provide improved pervious lowdensity carbon fiber reinforced composite articles exhibiting a bulkdensity of about 0.4 to 1.4 grams/cc It is a further object of theinvention to provide pervious low density carbon fiber reinforcedcomposite articles which exhibit excellent mechanical properties, andwhich are particularly suitable for use as facing sheets of an acousticsandwich liner which serves as a noise suppression function in aturbofan nacelle for a jet engine.

These and other objects as well as the scope, nature, and utilization ofthe invention will be apparent from the following detailed descriptionand appended claims.

DESCRIPTION OF THE DRAWINGS FIG. 3 is an enlarged perspective view of asection of the pervious low density carbon fiber reinforced compositearticle of FIG. 2.

SUMMARY OF THE INVENTION It has been found that an improved pervious lowdensity carbon fiber reinforced composite article may be formed by:

a. providing a continuous length of a fibrous open weave tape comprisinga plurality of adjacent substantially parallel and laterally spacedlinear warp ends of an organic polymeric fibrous material capable ofundergoing conversion to a carbonaceous fibrous material substantiallycoextensive with the length of the tape wherein no substantial lateralcontact is made between the adjacent warp ends, and a fibrous weft pickinterlaced with the warp ends in a plain weave construction at afrequency of about 2 to 8 picks per inch of said tape with the weft pickbeing provided under a tension sufficient that the linear configurationof the warp ends is substantially unimpaired,

b. continuously passing the tape in the direction ofits length through aseries of heating zones while substantially suspended therein to form afibrous tape which contains at least 90 per cent carbon by weight whilepreserving an open weave construction within the resulting tape whereinno substantial contact is made between the laterally adjacent warp ends,

c. impregnating the tape with a thermosetting resinous material whilesubstantially maintaining lateral intersticcs between the adjoining warpends within a given layer of the tape, and

d. substantially curing the thermosetting resinous material while inintimate association with at least one layer of the tape to form apervious low density composite article.

In a preferred embodiment the warp ends of the tape prior to passagethrough the heating zones are formed from an acrylic polymer (eg anacrylonitrile homopolymer). Additionally, a plurality of layers or pliesof the resulting carbon tape are preferably superimposed within thecomposite article to form a composite article of increased thickness.

DESCRIPTION OF PREFERRED EMBODIMENTS The precursor tape which isconverted to a carbonaceous fibrous material comprises a plurality ofadjacent substantially parallel and laterally spaced linear warp endssubstantially coextensive with the length of the tape. The tapepossesses a plain weave construction (described in detail hereafter)wherein each weft pick passes successively over and under each warp end.

The warp ends are composed of an organic polymeric fibrous materialcapable of conversion to a carbonaceous fibrous material. The warp endsmay be conveniently selected from those fibrous materials which arerecognized as being suitable for thermal conversion to a carbonaceousfibrous material. For instance, the warp ends may be derived fromorganic polymers such as an acrylic polymer, a cellulosic polymer, apolyamide, a polybenzimidazole, polyvinyl alcohol, pitch, etc. Asdiscussed hereafter, acrylic polymeric materials are particularly suitedfor use in the formation of the warp ends employed in the presentprocess. Illustrative examples of suitable cellulosic materials includethe natural and regenerated forms of cellulose, e.g. rayon. Illustrativeexamples of suitable polyamide materials include the aromaticpolyamides, such as nylon 6T, which is formed by the condensation ofhexamethylenediamine and terephthalic acid. An illustrative example of asuitable polybenzimidazole is poly-2,2-mphenylene-5,5-bibenzimidazole.

The acrylic polymeric material prior to thermal stabilization is formedprimarily of recurring acrylonitrile units. For instance, the acrylicpolymer should contain not less than about mol per cent of acrylonitrileunits with not more than about 15 mol per cent of a monovinyl compoundwhich is copolymerizable with acrylonitrile such as styrene, methylacrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidenechloride, vinyl pyridine, and the like, or a plurality of such monomers.A particularly preferred acrylic polymeric material is an acrylonitrilehomopolymer, or a closely related acrylonitrile copolymer (i.e. containsat least about mol per cent of acrylonitrile units and up to about 5 molper cent of one or more monovinyl compounds copolymerized withacrylonitrile).

The warp ends may be provided in a variety of physical configurations.For instance, the warp ends may assume the configuration of continuouslengths of multifilament yarns, tows, strands, cables, or similarfibrous assemblages. In a preferred embodiment of the process the warpends are a continuous multifilament yarn.

For example, the laterally spaced warp ends are preferably a continuousmultifilament yarn (e.g. an acrylic yarn) having a total denier of about80 to 10,000 (preferably about 500 to 4,000), and a denier per filamentof about 1 to 3 (preferably about 2), which are provided in theprecursor tape at a frequency of about 4 to 200 (preferably about 7 to20) ends per inch of tape width.

The warp ends may optionally be provided with a twist which tends toimprove the handling characteristics. For instance, a twist of about 0.1to 5 tpi, and preferably about 0.3 to 1.0 tpi, may be utilized. Also, afalse twist may be used instead of or in addition to a real twist.Alternatively, one may select bundles of fibrous material which possessessentially no twist.

The warp ends may be drawn in accordance with conventional techniques inorder to improve their orientation. For instance, acrylic warp ends maybe preliminarily drawn by stretching before or after incorporation inthe tape while in contact with a hot shoe at about to C. Additionalrepresentative drawing techniques are disclosed in U. S. Pat. Nos.2,455,173; 2,948,581; and 3,122,412. It is recommended that acrylic warpends selected for use in the process be initially drawn to a singlefilament tenacity of at least about 3 grams per denier. If desired,however, the warp ends may be more highly oriented, e.g. drawn up to asingle filament tenacity of about 7.5 to 8 grams per denier, or more.

The weft pick of the precursor tape is preferably also composed of anorganic polymeric fibrous material which is capable of undergoingcarbonization without the destruction of its original fibrousconfiguration. If desired, however, the weft pick may be initiallyprovided as a previously stabilized organic polymeric fibrous material.For example, the weft pick may be initially provided as a previouslystabilized (e.g. preoxidized) acrylic fibrous material.

The fibrous weft pick may be provided in a variety of physicalconfigurations. For instance, the weft pick may assume the configurationof a multifilament yarn,

tow, strand, cable, or similar assemblage. In a preferred embodiment ofthe process the weft pick (e.g. a continuous multifilament yarn) has atotal denier equal to or preferably less than that of the warp ends(e.g. continuous multifilament yarn warp ends). The total denier for theweft pick accordingly may range from about 40 to 5,000. Preferably thetotal denier of a multifilament acrylic yarn weft pick prior to thermalstabilization is below about 400, e.g. about 40 to 350, total denier. Ina particularly preferred embodiment of the process the total denier ofthe weft pick is about 0.2 to 0.5 times the total denier of a warp end.A minor amount of twist may be beneficially provided in a multifilamentyarn weft pick which improves the handling characteristics duringweaving. For instance, the weft pick may be provided with a twist ofabout 0.1 to 5 tpi (preferably 0.1 to 3 tpi), and most preferably about0.2 to 0.7 tpi. if a twist is utilized in the warp ends, it isrecommended that any twist employed in the weft pick be to a lesserdegree so that the weft pick may readily assume a more flatenedconfiguration when in contact with warp ends.

It is essential that the weft pick utilized in the formation of the tapelacks a tendency to undergo excessive shrinkage during heat treatment(described hereafter) which would eliminate a laterally spacedrelationship of the adjacent warp ends, impart a pucker (i.e. nonlinearconfiguration) to the warp ends, or interfere with the flatconfiguration of the tape. In a preferred embodiment of the process theweft pick is hot drawn at least about 3 times its as-spun length toincrease its orientation and is subsequently relaxed (e.g. 5 to 40 percent of drawn length) prior to incorporation in the precursor tape sothat its tendency to undergo shrinkage is minimized.

The fibrous material utilized as the warp ends and weft pick mayoptionally be provided in intimate association with one or morecatalytic agents capable of enhancing the rate of the thermal conversionto a carbonaceous fibrous material.

The fibrous open weave tape utilized as the precursor is provided in aplain open weave construction which is unbalanced in the sense that thenumerical proportion of warp ends to weft picks per square inch presentwithin the same is substantially greater than 1:1. Commonly the tapecomprises 50 to 200 adjacent warp ends; however, even a substantiallylarger number of warp ends can be employed, e.g. 500 or more. The warpends are substantially coextensive with the length of the tape. The weftpick present within the tape is provided at a frequency of about 2 to 8picks per inch of the tape (preferably 2 to 6 picks per inch of thetape), and is most preferably provided at a frequency of about 4 picksper inch of the tape. The optimum pick frequency selected is influencedby the total denier of the weft pick. For instance, if a pick frequencyas great as 8 is utilized the weft pick preferably does not possess atotal denier in excess of about 100. If the pick frequency is much lessthan about 2 picks per inch of the tape, then weave stability is notachieved and the desired lateral separation of adjacent warp ends isunattainable. If the pick frequency is much more than about 8 picks perinch, then there is a tendency for the weft pick to impart a crimp ornon'linear configuration to the warp ends which results in diminishedtape tensile properties in the longitudinal direction. Since the weftpick is provided at a relatively low frequency, and preferably as acontinuous length, it may intersect the edge of the tape at an angleother than exactly ninety degrees unlike common woven fabrics. The exactangle of intersection with the edge of the tape is influenced by thepick frequency, and the width of the tape (i.e. num her and total denierof the warp ends). If desired, the edge of the tape may optionally beremoved following weaving wherein the weft pick is cut into a pluralityof discrete lengths which traverse the parallel warp ends.

The plain weave construction of the precursor tape is such that theadjacent warp ends are spaced by the weft pick in such a manner that nosubstantial lateral contact is made between the same and a flat tape isproduced. The weft pick is provided under a tension sufficient that thelinear configuration of the warp inds present within the tape issubstantially unimpaired. Additionally, any crimp which is present inthe tape components should be present in the weft pick and not in thewarp ends.

The precursor tape of the open weave construction utilized in thepresent process can be formed employing conventional weaving equipmentas will be apparent to those skilled in weaving technology. Forinstance, the warp ends may be beamed in a laterally spaced manner, andthe weft pick subsequently inserted at appropriate intervals utilizing anarrow fabric shuttle loom. Care, of course, must be taken to insurethat the tension exerted upon the weft pick is insufficient to impairthe substantially linear configuration of the warp ends.

The heating temperatures, heating atmospheres, and residence timesutilized in the present process to produce carbon fibers may be inaccordance with thermal conversion techniques heretofore known in theart. The plurality of adjacent ends of an organic polymeric fibrousmaterial as well as the fibrous weft pick while in the form of a tape(as heretofore described) are converted to a carbonaceous fibrousmaterial by continuous passage in the direction of the tape lengththrough a series of heating zones while substantially suspended thereinto form a fibrous product which contains at least per cent carbon byweight. During the thermal conversion a substantially uniform lateralshrinkage of the tape of about 10 to 50 per cent based upon the originaltape width commonly occurs. However, throughout the thermal conversiontreatment an open weave construction of the tape is preserved with nosubstantial contact being made between the laterally adjacent warp ends.

The series of heating zones through which the open weave tape iscontinuously passed commonly include (1) a stabilization zone, and (2) acarbonization zone.

The stabilization heating zone is commonly provided at a temperature ofabout 200 to 400 C. depending upon the composition of the tape. As willbe apparent to those skilled in the art, the atmosphere provided in thestabilization heating zone may be varied. For instance, a cellulosicprecursor is commonly stabilized in (1) an oxygen-containing atmosphereor (2) in an inert or non-oxidizing atmosphere, such as nitrogen,helium, argon, etc. Additionally, precursors such as an acrylic polymer,a polyamide, a polybenzimidazole, or polyvinyl alcohol are commonlystabilized in an oxygencontaining atmosphere. Air may be convenientlyselected as the oxygen-containing atmosphere for use in the process.When the stabilization treatment is conducted in an oxygen-containingatmosphere, it is commonly termed a preoxidation" treatment.

The stabilization heating zone is substantially enclosed in order tofacilitate the confinement and withdrawal of off gases and/or themaintenance of an appropriate atmosphere. When a non-oxidizingatmosphere is desired within the heat treatment chamber, the tape maypass through a seal as it continuously enters and leaves the heattreatment chamber in order to exclude oxygen.

The stabilization of fibers of acrylonitrile homopolymers and copolymersin an oxygen-containing atmosphere involves (I) an oxidativecross-linking reaction of adjoining molecules as well as (2) acyclization reaction of pendant nitrile groups to a condenseddihydropyridine structure. While the reaction mechanism is complex andnot readily explainable, it is believed that these ends reactions occurconcurrently, or are to some extent competing reactions.

The cyclization reaction involving pendant nitrile groups which occursupon exposure of an acrylic fibrous material to heat is generally highlyexothermic and, if uncontrolled, results in the destruction of thefibrous configuration of the starting material. In some instances thisexothermic reaction will occur with explosive violence and result in thefibrous material being consumed by flame. More commonly, however, thefibrous material will simply rupture, disintegrate and/or coalesce whenthe critical temperature is reached. As the quantity of comonomerpresent in an acrylonitrile copolymer is increased, a fibrous materialconsisting of the same tends to soften at a progressively lowertemperature and the possible destruction of the original fibrousconfiguration through coalescence of adjoining fibers becomes a factorof increasing importance. The critical temperature referred to herein isdefined as the temperature at which the fibrous configuration of a givensample of acrylic fibrous starting material will be destroyed in theabsence of prior stabilization.

In a preferred embodiment of the invention the acrylic starting materialexhibits a critical temperature of at least about 300 C., e.g. about 300C. to 330 C. In addition to visual observation, the detection of thecritical temperature of a given acrylic fibrous material may be aided bythe use of thermoanalytical methods, such as differential scanningcalorimeter techniques, whereby the location and magnitude of theexothermic reaction can be measured quantitatively.

The stabilized acrylic warp ends l retain essentially the same fibrousconfiguration as the starting material, (2) are capable of undergoingcarbonization, (3) are black in appearance, (4) are non-burning whensubjected to an ordinary match flame, and (5) commonly contain a boundoxygen content of at least about 7 per cent by weight as determined bythe Unterzaucher analysis.

In a preferred embodiment of the process the open weave tape (heretoforedescribed) is stabilized in accordance with the processing conditions ofcommonly assigned U. S. Ser. Nos. 749,957, filed Aug. 8, 1968, and865,332, filed Oct. 10, 1969 (now abandoned) which are hereinincorporated by reference.

The carbonization heating zone is commonly provided with an inert ornon-oxidizing atmosphere at a temperature of at least about 900 C. (e.g.900 to 1,600 C.). Suitable inert atmospheres include nitrogen, argon,helium, etc. During the carbonization reaction elements present in thetape other than carbon, e.g. nitrogen, hydrogen and oxygen aresubstantially expelled until the warp ends contain at least per centcarbon by weight, and preferably at least per cent carbon by weight.

An optional graphitization zone is commonly provided with an inert ornon-oxidizing atmosphere at a more highly elevated temperature of about2,000 to 3,l00 C. In a preferred embodiment of the process a graphitizedopen weave tape is produced wherein the fiber of the warp ends exhibitsa Young's modulus of at least about 40,000,000 psi, and a tensilestrength of at least about 250,000 psi.

A longitudinal tension may optionally be applied to the tape whilepassing through the stabilization. carbonization and/or graphitizationheating zones in accordance with techniques known in the art.

In a preferred embodiment of the process the carbonization andgraphitization of a stabilized acrylic open weave tape may be conductedby the continuous passage of the same through a single heatingapparatus, such as the susceptor of an induction furnace, provided witha temperature gradient in accordance with the teachings of commonlyassigned U. S. Ser. No. 777,275, filed Nov. 20, 1968 (now abandoned),which is herein incorporated by reference. A particularly preferredsusceptor for use in the production of carbonaceous fibrous materialswhile in tape form is disclosed in commonly assigned U. S. Ser. No.46,675, filed June 16, 1970 (now US. Pat. No. 3,656,910), which isherein incorporated by reference.

The carbonaceous tape, whether formed of amorphous or graphitic carbon,can next optionally be passed through a surface treatment zone whereinits ability to bond to a thermosetting resinous matrix material isenhanced. Any conventional surface treatment technique may be selected.

As previously indicated, during the stabilization and carbonizationsteps of the present process it is common for the width of the tape todiminish due to controlled shrinkage as elements other than carbon areexpelled. A flat tape configuration is nevertheless retained, as well asan open weave construction wherein no substantial lateral contact ismade between adjacent warp ends.

The open weave tape undergoing treatment in the present process iscontinuously passed in the direction of its length through each of theheating zones (e.g. a stabilization zone and a carbonization zone). Ifdesired, the forward movement of the tape may be terminated betweenheating zones and the tape collected upon a support where it is storedprior to additional processing. It is recommended, however, that theheating zones be aligned in close proximity and the tape continuouslypassed from one zone to another without termination of the forwardmovement. Various rolls, or other guides may be employed to direct themovement of the tape as will be apparent to those skilled in fibertechnology.

The resulting carbon tape is next impregnated with a thermosettingresinous material while maintaining lateral interstices between adjacentwarp ends within a given layer of the tape. The impregnation step may beconducted on either a continuous or a batch basis. For instance, thecarbon tape may be continuously passed through an impregnation orcoating zone wherein the thermosetting resinous material is continuouslyapplied, or segments or layers of the tape may be impregnated whilestatically situated. A single layer of open The thermosetting resinousmaterial is applied to the carbon tape while in a liquid consistency,and may be applied from either a solvent or a solventless system. Freelyflowing dilute (i.e. highly cut) or low viscosity resin systems arepreferred. Preferred impregnation techniques include the immersion ofthe carbon tape in a resin bath. The preservation of lateral intersticesbetween adjacent warp ends within a given layer of carbon tape may beenhanced by the passage of a current of gas therethrough immediatelyfollowing application of the liquid thermosetting resinous material. Forinstance, the current of gas may be passed therethrough upon withdrawalfrom the resin bath.

Representative thermosetting resinous materials which may be utilized inthe formation of the composite articles include: epoxy resins, phenolicresins, polyester resins, polyimide resins, polybenzimidazoles,polyurethanes, etc. The preferred thermosetting resinous material is anepoxy resin or an aromatic polyimide resin.

The epoxy resin utilized as the resinous impregnation or matrix materialmay be prepared by the condensation of bisphenol A (4,4 isopropylidenediphenol) and epichlorohydrin. Also, other polyols, such as aliphaticglycols and novolak resins (e.g., phenol-formaldehyde resins), acids orother active hydrogen containing compounds may be reacted withepichlorohydrin for the production of epoxy resins suitable for use asthe resinous matrix material. Epoxy resins are preferably selected whichpossess or can be modified to possess the requisite flow characteristicsprior to curing. Numerous reactive diluents or modifiers which arecapable of increasing the flow properties of uncured epoxy resins arewell known and include butyl glycidyl ether, higher molecular weightaliphatic and cycloaliphatic monoglycidyl ethers, styrene oxide,aliphatic and cycloaliphatic dicylcidyl ethers, and mixtures of theabove.

In preferred embodiments of the invention, epoxy resins are selected toserve as the resinous impregnation material which possess terminalepoxide groups and are the condensation product of bisphenol A andepichlorohydrin of the following formula:

where n varies between zero and a small number less than about 10. Whenn is zero, the resin prior to curing is a very fluid light-coloredmaterial which is essentially the diglycidyl ether of bisphenol A. Asthe molecular weight increases so generally does the viscosity of theresins. Accordingly, particularly preferred liquid epoxy resinsgenerally possess an n value averaging less than about 1.0. Illustrativeexamples of standard trade designations of particularly usefulcommerically available epoxy resins include: Epi-Rez 508, and Epi-Rez510 (Celanese Coatings), ERLA 2256 (Union Carbide), ERLA 4617 (UnionCarbide), and Epon (Shell) epoxy resins.

Epoxy novolak resins formed by the reacting of epichlorohydrin withphenol-formaldehyde resins are also particularly preferred thermosettingresins. An illustrative example of a highly useful resin is Epi-Rez 5I55 epoxy novolak resin (Celanese Coatings).

A variety of epoxy resin curing agents may be employed in conjunctionwith the epoxy resin. The curing or hardening of the epoxy resintypically involves further reaction of the epoxy or hydroxyl groups tocause molecular chain growth and cross-linking. The term curing agent asused herein is accordingly defined to include the various hardeners ofthe co-reactant type. Illustrative classes of known epoxy curing agentswhich may be utilized include aliphatic and aromatic amines, polyamides,tertiary amines, amine adducts, acid anhydrides, acids, aldehydecondensation products, and Lewis acid type catalysts, such as borontrifluoride. The preferred epoxy curing agents for use with the epoxyresin are acid anhydrides (e.g., hexahydrophthalic acid andmethylbicyclo[2.2.l ]heptene-2,2-dicarboxylic anhydride isomers marketedunder the designation Nadic Methyl Anhydride by the Allied ChemicalCompany), and aromatic amines (e.g., meta-phenylene diamine anddimethylaniline).

In preferred embodiments, aromatic polyimide resins are selected toserve as the resinous impregnation material which as is known in the artare produced essentially in two steps by the reaction of a dianhydridewith a diprimary aromatic amine. In the first step of the reaction, apolyamic acid is formed which may be subsequently converted by heat orthrough the use of suitable catalysts and water acceptors tohigh-molecularweight polyimides. The resulting aromatic polyimides maybe either linear or cross-linked. When both precursors are aromatic,impregnation polymers of improved heat resistance are developed.

Representative dianhydride reactants for use in the formation of thearomatic polyimides include:

pyromellitic dianhydride (PMDA);

2,3,6,7-napthalenetetracarboxylic acid dianhydride;

3,3',4,4'-diphenyltetracarboxylic acid dianhydride;

l,2,5,6-napthalenetetracarboxylic acid dianhydride;

2,2',3,3-diphenyltetracarboxylic acid dianhydride;

thiophene-2,3,4,5-tetracarboxylic acid anhydride;

2,2-bis( 3 ,4-biscarboxyphenyl)propane dianhydride (PPDA);

3,4-dicarboxyphenyl sulfone dianhydride;

perylene-3,4,9, l O-tetracarboxylic acid dianhydride;

V bis(3,4-dicarboxyphenyl) ether dianhydride (PEDA);

ethylenetetracarboxylic acid dianhydride; and3,4,3,4-benzophenonetetracarboxylic dianhydride (BTDA).

Representative diamine reactants for use in the formation of thearomatic polyimides include:

m-phenylenediamine(MPD); p-phenylenediamine (PPD);2,2-bis(4-aminophenyl)propane (DDP); 4,4'-methylenedianiline (DDM);benzidine (PP); 4,4'-diaminodiphenyl sulfide (PSP);'4,4'-diaminodiphenyl sulfone (PSO P);

4,4'-diaminodiphenyl ether (POP); 1,5-diaminonaphthalene;3,3'-dimethylbenzidine;

3 ,3 '-dimethoxybenzidine;

2,4-bis(B-amino-tert-butyl)toluene;

bis(4-B-amino-tert-butyl phenyl) ether;

1,4-bis( 2-methyl-4-aminopentyl)benzene;

1-isopropyl-2,4-phenylenediamine;

m-xylylenediamine;

p-xylylenediamine;

di(4-aminocyclohexyl)methane;

hexamethylenediamine;

heptamethylenediamine;

octamethylenediamine;

nonamethylenediamine;

decamethylenediamine;

oxydianiline;

3-methylheptamethylenediamine;

4,4-dimethylheptamethylenediamine;

2,1 l-diaminododecane;

l,2-bis( 3-aminopropoxyethane 2,2-dimethylpropylenediamine;

3-methoxyhexamethylenediamine; 2,5-dimethylhexamethylenediamine;2,5-dimethylheptamethylenediamine; 3-methylheptamethylenediamine;1,4-diaminocyclohexane;

1,1 Z-diaminooctadecane;

bis(3-aminopropyl)sulfide; and

bis(3-aminopropyl)methylamine.

An illustrative example by standard trade designation of a particularlyuseful commercially available polyimide resin is Pyralin 4707 polyimide(DuPont).

Representative aromatic polyimide formation techniques are disclosed inChapter 8 of New Linear Polymers, by Henry Lee et a1. (McGraw-Hill,1967), U.S. Pat. Nos. 3,179,630; 3,179,631; 3,179,632; 3,179,633;3,179,634, and 3,558,350. Each of these disclosures is hereinincorporated by reference.

The quantity of therrnosetting resinous material applied to the carbontape is adjusted so that upon curing the thermoset resinous material isin intimate association with at least one layer of the tape in aconcentration of about 20 to 50 per cent by weight based upon the totalweight of the resulting composite article, and preferably in aconcentration of about 25 to 40 per cent by weight.

The thermosetting resinous material may be cured in accordance withconventional curing procedures for the particular thermosetting resinsystem. Prior to curing a plurality of layers or plies of the carbontape which were individually or jointly impregnated may be superimposedto form a composite article of increased thickness. For instance, two ormore (e.g. up to 200, or more) layers or plies of the impregnated openweave carbon tape may be stacked in a unidirectionally aligned patternor a multidirectionally aligned pattern (e.g. at right angles). Uponsubsequent curing a unitary pervious low density composite article isformed. In FIG. 3 is illustrated an enlarged perspective view of acomposite article formed in accordance with the present inventionwherein four layers of the impregnated open weave carbon tape weredisposed at right angles.

The resulting composite articles formed in accordance with the presentinvention exhibit a pore volume or open structure which amounts toapproximately 10 to 75 per cent by volume of the total compositearticle, and preferably about 50 to 70 per cent by volume. Suchcomposite articles additionally exhibit a bulk density of about 0.4 to1.4 grams/cc, and preferably a bulk density of about 0.5 to 1.0 grams/ccThe composite articles of the present invention find applicability inthose areas where extremely lightweight stiff structural elements arerequired, e.g. in aircraft structural applications. The compositearticles exhibit excellent mechanical properties, and are particularlysuited for use as facing sheets of an acoustic sandwich liner whichserves as a noise suppression function in a turbofan nacelle for a jetengine. The composite articles may also be formed into honeycomb corestructural elements.

The following examples are provided as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples.

In the examples highly unbalanced tapes of various plain open weaveconstructions were continuously passed in the direction of their lengththrough (1) a pretreatment zone, (2) a stabilization zone, and (3) aheating zone provided with a temperature gradient wherein bothcarbonization and graphitization were carried out. Following resinimpregnation composite articles were formed incorporating the resultinggraphite tape as fibrous reinforcement.

Each tape was produced by initially beaming 60 warp ends of a dry spunacrylonitrile homopolymer, and inserting a weft pick by use of aFletcher narrow fabric loom shuttle loom. Each warp end consisted ofabout 385 continuous filaments having a total denier of about 775, andwas provided with a twist of about 0.5 turn per inch. The 60 warp endswere aligned in an adjacent substantially parallel and laterally spacedconfiguration to form a flat tape having a width of 3 inches. Prior toincorporation in the tape the warp ends had been hot drawn to a singlefilament tenacity of about 4 grams per denier.

The pretreatment of the acrylonitrile homopolymer tape was conducted inaccordance with the teachings of commonly assigned U.S. Ser. No. 17,962,filed Mar. 9, 1970 (now abandoned). The tape was continuously passedthrough an oven containing circulating air provided at 220 C. whileunder a longitudinal tension sufficient to permit a 16 per centreduction in length brought about by shrinkage for a residence time ofabout 300 seconds.

The stabilization (i.e. preoxidation) was conducted in accordance withthe teachings of commonly assigned U.S. Ser. No. 865,332, filed Oct. 10,1969 (now abandoned). The tape was continuously passed through an ovencontaining circulating air maintained at about 270 C. while under alongitudinal tension sufficient to maintain a constant length for aresidence time of about minutes. The preoxidized open weave tape wasblack in appearance, retained its initial fibrous configurationessentially intact, was non-burning when subjected to an ordinary matchflame, and contained a bound oxygen content of 10 per cent by weight asdetermined by the Unterzaucher analysis.

The preoxidized tape was continuously passed through a heating zone ofan induction furnace provided with a circulating nitrogen atmosphere anda temperature gradient in accordance with the teachings of commonlyassigned U. S. Ser. No. 777,275, filed Nov. 20, 1968 (now abandoned).The hollow graphite susceptor of the induction furnace was formed inaccordance with the teachings of commonly assigned U.

5. Ser. No. 46,675, filed June 16, 1970 (now US. Pat. No. 3,656,910).The temperature gradient within the heating zone raised the tape fromroom temperature (i.e. about 25 C.) to a temperature of 800 C. inapproximately 50 seconds after entering the susceptor, from 800 C. tol,600 C. in approximately 25 seconds to produce a carbonized tape, andfrom 1,600 C. to 2,900 C. in approximately 50 seconds where it wasmaintained i 50 C. for about 40 seconds to produce a graphitized tape. Alongitudinal tension of 20 pounds (i.e. about 150 grams per warp end)was exerted upon the tape as it passed through the heating zone of theinduction furnace. The warp ends and weft picks substantially retainedtheir original fibrous configuration following carbonization andgraphitization and exhibited a specific gravity of about 2.0. The tapeexhibited a predominant X -ray diffraction pattern characteristic ofgraphitic carbon when subjected to X-ray analysis, contained in excessof 99 per cent carbon by weight, and retained an open weave constructionwherein no substantial contact was made between the laterally spacedwarp ends. The Youngs modulus and tensile strength of the graphite warpends were determined.

The resulting graphite tape was next impregnated with an aromaticpolyimide resin system by continuous passage for a residence time ofabout seconds through a liquid resin bath provided at 25 C. Thethermosetting resin was commercially available Pyralin 4707 polyimide(DuPont) which was diluted with acetone to form a liquid resin systemhaving a composition of 3 parts acetone by weight, and parts by weightof the commerically available resin. The commercial resin had a 45 percent by weight solids system which was convertible to an aromaticpolyimide, and a solvent mixture of 2 parts by weight ofN-methylpyrrolidine, and 1 part by weight of xylene.

Composite articles of 3 X 10 inches and 0.016 inch thickness were nextformed by superimposing four previously impregnated plies of the tape asillustrated in FIGS. 2 and 3. Each ply was arranged at 90 to eachadjoining ply. The composites were formed by a vacuum bag techniquewherein the four resin impregnated plies were stacked in sequence, andwere placed in a polyimide film bag (i.e. a Kepton polyimide film) witha bleeder cloth placed on one side of the stacked plies. The contents ofthe bag were subjected to a vacuum level of 24 in. Hg. while heated fromroom temperature (i.e. about 70 F.) to 260 F. over a 45 minute period,from 260 F. to 310 F. over a 180 minute period, and from 310 F. to 365F. over a minute period where they were maintained for a 45 minuteperiod prior to cooling to room temperature (i.e. about 70 F.) over a 3hour period. The cooling step was also conducted at a vacuum level of 24in. Hg.

The bulk density, pore volume, modulus, and tensile strength for theresulting composites were determined by use of the following techniques.

The bulk density was computed as the weight of the composite articledivided by the apparent volume of the same.

The percentage pore volume was computed by subtracting the weight of thepervious composite article from the weight of a corresponding solidcomposite article, dividing by the weight of the corresponding solidcomposite article, and multiplying by 100.

The modulus was computed from the initial slope of the stress straincurve obtained by standard tensile test procedures.

The tensile strength was computed from the breaking load of thecomposite article determined by the tensile test procedure divided bythe apparent cross-sectional area of the composite article.

EXAMPLE 1 An acrylonitrile homopolymer tape having a plain weaveconstruction as illustrated in part in FIG. 1 was employed.Representative warp ends are identified at A and representative weftpicks at B. The weft pick was formed from approximately continuous filsof acrylonitrile homopolymer having a total denier of about 200 and atwist of 0.5 turn per inch. The weft pick was provided at a frequency of4 picks per inch of tape.

The width of the tape following graphitization was reduced to about 1.5inches. The warp density of the tape following graphitization wasreduced from a density of 20 ends per inch in the precursor tape to 40ends per inch. The linear configuration of the spaced substantiallyparallel warp ends was retained. The warp ends exhibited a Youngsmodulus of about 70,000,000 psi and a tensile strength of about 300,000psi.

The resulting four-ply composite article of FIG. 3 contained the curedthermosetting resin in a concentration of about 30 per cent by weightbased upon the total weight of the composite, exhibited a bulk densityof 0.82 grams/cc, a pore volume of 50 per cent, a modulus of 11,000,000psi, and a tensile strength of 50,000 psi.

EXAMPLE II An acrylonitrile homopolymer tape having a plain weaveconstruction identical to that described in Example was employed whereinthe acrylonitrile homopolymer weft pick prior to formation of the tapehad been previously stabilized. More specifically, the weft pick hadbeen stabilized on a continuous basis while in yarn form by continuouspassage for 60 minutes through a heating zone provided at 270 C.

The width of the tape following graphitization was reduced to about 2inches. The warp density of the tape following graphitization wasreduced from a density of 20 ends per inch in the precursor tape to 30ends per inch. The linear configuration of the spaced substantiallyparallel warp ends was retained. Prior to surface treatment the warpends exhibited a Youngs modulus of about $0,000,000 psi and a tensilestrength of about 300,000 psi.

The resulting four-ply composite article contained the curedthermosetting resin in a concentration of about 25 per cent by weightbased upon the total weight of the composite, exhibited a bulk densityof 0.6 grams/cc, a pore volume of 69 per cent, a modulus of 7,000,000psi, and a tensile strength of 35,000 psi.

EXAMPLE Ill Example 1 was repeated with the exception that thethermosetting resinous material was an epoxy resin rather than apolyimide and a different curing cycle was employed. More specifically,the epoxy resin system comprised approximately 200 parts by weight of acondensation product of bisphenol A and epichlorohydrin, 70.8 parts byweight diamino-diphenyl sulfone curing agent, and 300 parts by weight ofacetone solvent. The

curing was conducted by the vacuum bag technique previously described.The contents of the bag were subjected to a vacuum level of 24 in. Hg.while heated from room temperature (i.e. about 70 F.) to 150 F. wherethey were maintained for minutes, from 150 F. to 350 F. over a 60 minuteperiod where they were maintained for a 90 minute period prior tocooling to room temperature (i.e. about 70 F.) over a 2 hour period. Thecooling step was also conducted at a vacuum level of 24 in. Hg.

The resulting four-ply composite article contained the curedthermosetting resin in a concentration of about 25 per cent by weightbased upon the total weight of the composite, exhibited a bulk densityof 0.52 grams/cc, a pore volume of 68 per cent, a modulus of 8,200,000psi, and a tensile strength of 40,000

psi.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations andmodifications are to be considered within the purview and scope of theclaims appended hereto.

1 claim:

1. A process for the production of a pervious low density carbon fiberreinforced composite article comprising:

a. providing a continuous length of a fibrous open weave tape comprisinga plurality of adjacent substantially parallel and laterally spacedlinear warp ends of an organic polymeric fibrous material capable ofundergoing conversion to a carbonaceous fibrous material substantiallycoextensive with the length of said tape wherein no substantial lateralcontact is made between said adjacent warp ends, and a fibrous weft pickinterlaced with said warp ends in a plain weave construction at afrequency of about 2 to 8 picks per inch of said tape with said weftpick being provided under a tension sufficient that said linearconfiguration of said warp ends is substantially unimpaired,

b. continuously passing said tape in the direction of its length througha series of heating zones while substantially suspended thereincomprising heating in a carbonization zone containing a non-oxidizingatmosphere at a temperature of at least about 900 C. to form a fibroustape which contains at least 90 per cent carbon by weight whilepreserving an open weave construction within said resulting tape whereinno substantial contact is made between said laterally adjacent warpends,

c. impregnating said tape with a thermosetting resinous material whilesubstantially maintaining lateral interstices between said adjacent warpends within a given layer of said tape, and

d. substantially curing said thermosetting resinous material while inintimate association with at least one layer of said tape to form apervious low density composite article containing said thermosetresinous material in a concentration of about 20 to 50 per cent byweight based upon the total weight of said composite article, having apore volume of approximately 10 to per cent by volume of the totalcomposite article, and a bulk density of about 0.4 to 1.4 grams/cc 2. Aprocess in accordance with claim 1 wherein said warp ends of said tapeprior to passage through said series of heating zones are an acrylicfibrous material containing at least about mol per cent of acrylonitrileunits and up to about 15 mol per cent of one or more monovinyl unitscopolymerized therewith.

3. A process according to claim 1 wherein said warp ends of said tapeprior to passage through said series of heating zones are anacrylonitrile homopolymer.

4. A process according to claim 1 wherein the composition of said warpends of said tape prior to passage through said series of heating zonesis substantially identical to that of said weft pick.

5. A process according to claim 1 wherein said weft pick of said tapeprior to passage through said series of heating zones is a previouslystabilized acrylic fibrous material.

6. A process according to claim 1 wherein said heating zones comprise astabilization zone, and a carbonization zone.

7. A process according to claim 1 wherein said tape following passagethrough said heating zones contains at least about per cent carbon byweight and exhibits a predominant graphitic X-ray diffraction pattern.

8. A process according to claim 1 wherein said thermosetting resinousmaterial upon curing is an aromatic polyimide.

9. A process according to claim 1 wherein the formation of lateralinterstices between said adjacent warp ends within a given layer of saidtape is enhanced by the passage of a current of gas therethroughfollowing impregnation with said thermosetting resinous material.

2. A process in accordance with claim 1 wherein said warp ends of said tape prior to passage through said series of heating zones are an acrylic fibrous material containing at least about 85 mol per cent of acrylonitrile units and up to about 15 mol per cent of one or more monovinyl units copolymerized therewith.
 3. A process according to claim 1 wherein said warp ends of said tape prior to passage through said series of heating zones are an acrylonitrile homopolymer.
 4. A process according to claim 1 wherein the composition of said warp ends of said tape prior to passage through said series of heating zones is substantially identical to that of said weft pick.
 5. A process according to claim 1 wherein said weft pick of said tape prior to passage through said series of heating zones is a previously stabilized acrylic fibrous material.
 6. A process according to claim 1 wherein said heating zones comprise a stabilization zone, and a carbonization zone.
 7. A process according to claim 1 wherein said tape following passage through said heating zones contains at least about 95 per cent carbon by weight and exhibits a predominant graphitic X-ray diffraction pattern.
 8. A process according to claim 1 wherein said thermosetting resinous material upon curing is an aromatic polyimide.
 9. A process according to claim 1 wherein the formation of lateral interstices between said adjacent warp ends within a given layer of said tape is enhanced by the passage of a current of gas therethrough following impregnation with said thermosetting resinous material. 